JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

ciskender
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JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

« on: March 05, 2012, 01:55:52 PM »

Hi,
I have following problem with the JCM 800 2204 which was built exactly to the Ceriatone 2204 schematic.

I measure 390 V plate voltage and biased 37 mA (37 mV on 1 Ohm Resistor), them amp sounds good on low volumes.
However when I try to crank up, (gain and master at 12 o'clock) plate voltage drops 300 V and EL34s draws 100 mA, which is far beyond the tubes rated wattage.

I used oscilloscope, signal generator and dummy 8 Ohm load to see if its crank.
I applied 100-150 mA 1 kHz sinus signal to input and try to see clipped signal on the output.
Keep gain at 9 oclock (3 rd tick) and roll up the master volume up to 12 and far, I do not see any clipping on the output sinus, but as I said plate voltage drops 300 V and EL34s draws 100 mA.

Also another question, there is a very big volume difference between high and low inputs.
low input gives same volume level at gain and master set at 12 o'clock
where high input gain and master set at 3 (9 o'clock).
Is that volume difference normal?
Lo input volume is so low that even both gain and master set at 12 o'clock it is a bedroom level.


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cmoore
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Re: JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

« Reply #1 on: March 06, 2012, 03:44:22 AM »

Did this amp ever work correctly.?
Or is this a new build that has problems.?
Best


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ciskender
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Re: JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

« Reply #2 on: March 06, 2012, 09:19:48 AM »

Hi,

No this is new build, actually I built two of them, both are the same with different brand of tubes.

Before I built another amp (18 Watter 57 ) which works good.

I am not familiar with JCM 800 2204. I am not sure if this is the normal condition or not.
I newer crank up with speaker, the Hi input is too loud. This is why I used an oscilloscope and dummy load to see the clipping.
I checked voltages of tubes and compared with other sources, they seem ok.

Anyway the plate voltage drop and current consumption of tubes are suspicious on high volumes.
Or is it normal?

I checked my wiring couple of times but I could not find a problem.


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wyatt
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Re: JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

« Reply #3 on: March 06, 2012, 02:49:06 PM »

Well, the Low channel being at Noon on both Volumes should NOT be bedroom level. That's not right.

There should be a volume difference between the two channels. The way the 2203/2204 works is that the LOW circuit is very Plexi/JPM-esque; it has a similar circuit path to the 1987/1959, but with the added Master Volume. The HIGH input runs the signal through an additional gain stage before feeding it through the LOW circuit; the HIGH input like adding a boost pedal in front of the LOW input. It's a variation on the old mod of cascading the channels. More gain=more signal strength=more volume unless you choke it off at the MV. The JCM800 isn't a two channel amp...it's an either/or amp. You can choose either HIGH or LOW...you can't plug into both channels at the same time, even with a A/B/Y switcher, becuase they both use the same circuit path, the HIGH just adds an extra triode in front.

As for the rest...any amp's plate voltage is reduced as you crank it up and demand more current draw. That's why we have screen resistors to control the screen current if the plate voltage drops below the screen voltage. Is yours normal? Not sure. Obviously there is something wrong. Hard to troubleshoot amps online and I don't have a 800 around to check.


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ciskender
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Re: JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

« Reply #4 on: March 07, 2012, 08:44:03 AM »

Ok. Thank you very much.

The information "Low channel being at Noon on both Volumes should NOT be bedroom level" is a starting point for me.
So I guess there is a wrong wiring, I will check it again and post my findings.

Most probably I made a mistake on power amp or bias section.

Thanks a lot.


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ciskender
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Re: JCM 800 2204 Bias drift and B+ Voltage drop, never cranks up

« Reply #5 on: March 10, 2012, 09:40:46 PM »

I forgot to say, I used HALF POWER Mod.
http://www.ceriatone.com/images/mods/JCM800_2204CeriatoneHalfPow.jpg

I discovered that this mod schematic is different from "Two-EL34-Half-Power mod" in general mods section.
http://www.ceriatone.com/images/mods/Two-EL34-Half-Power.jpg

I revert back to original schematic without mod but I see that, there is no phase difference between LO input and speaker output but there is 180 degree phase difference between HI input and speaker output.
May be this is the case with half power mod, I did not check it before.

Also, with Hi input when I turn master volume up , I see big oscillation after half way.
Lo input behaves different, when master volume is half way open there is periodic squeal coming from somewhere propably from choke or some components on the board.

I suspect a faulty capacitor, I will try to find it.


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